This invention relates to a counterelectrode for a smart window based upon electrochromism, and the electrochromic device or smart window having the counterelectrode as one of its constituent elements.
In recent years, the development in an electrochromic element based upon electrochromism, referred to herein as a smart window, is proceeding briskly. The smart window usually comprised of an electrochromic material and an electrolyte sandwiched between a transparent electrode and a counterelectrode, is a device which changes color with good color contrast on current conduction and current interruption. That is, when the current is supplied to the smart window, its electrochromic material is colored and, once it is colored, it is not bleached, that is it does not lose color, and can only be bleached when the current is caused to flow in the reverse direction. The smart window is expected to be used in future in a variety of applications, because it is not in need of the electric power except during coloring and during bleaching, and can be controlled more easily than a liquid crystal display device.
An electrochromic material for the smart window which is colored by reduction, such as WO.sub.3, MoO.sub.3, V.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or TiO.sub.2, and an electrochromic material which is colored by oxidation, such as Cr.sub.2 O.sub.3, MnO.sub.2, CoO or NiO, have been employed. If such electrochromic material is used as one of the electrodes, it is required that the other electrode mounted facing the above-mentioned electrode with the electrolyte in-between, be transparent or light-transmitting or be colored in a complementary manner when driving the device in operation.
If an ITO glass commonly used as a transparent electrode of a common smart window is used as a counterelectrode of the smart window, the electrochromic efficiency becomes insufficient because of a smaller electrochemical capacity per unit area. If the electrical voltage applied across the electrodes is raised for improving the electrochromic efficiency, the cycle life of the coloration and bleaching cycles of the smart window is diminished.
Also, with a view to producing a smart window having a high electrochromic efficiency, it has been proposed in, for example, Japanese Laid-Open Patent Publications Nos. 62-151826 (1987), 62-204235 (1987) and 1-226122 (1989) to make combined use of an electrochromic material colored by reduction and an electrochromic material colored by oxidation within the inside of one and the same smart window.
However, none of the above-described prior-art materials or methods is as yet fully satisfactory as to the cycle life of the coloration and bleaching cycles under conditions of the higher electrochromic efficiency.